A bag contains $9$ red marbles, $10$ green marbles, and $10$ blue marbles. If a marble is randomly chosen, what is the probability that it is not red?
There are $9 + 10 + 10 = 29$ marbles in the bag. There are $9$ red marbles. That means $29 - 9 = 20$ are not red. The probability is $ \frac{20}{29}$.